As oil and gas offshore exploration and production operations are increasingly established in deeper waters, it has become more common for drilling activities to be performed from rigs that float on the surface of the water, such as drilling vessels or semi-submersible drilling rigs. Unlike fixed rigs or jack-up rigs, floating rigs are subject to wave motion, causing up- and down motion, which must be compensated for during drill, well completions, well interventions and other operations. Wave motion is of particular concern during “locked-to-bottom” operations (i.e. well completion, well testing and well intervention) where a landing string is physically connected to the wellhead at the seabed. Loss of heave compensation can lead to severe consequences.
Apart from the operational difficulties arising from the up-and-down motion of the floating rig, significant safety issues also arise, in particular the potential for the landing string to fracture or buckle and cause a blowout. Indeed, safety standards in offshore operations demand that a heave compensation system be regarded as an essential component of a floating rig during locked-to-bottom operations.
Known heave compensation systems may be described as employing passive heave compensation or active heave compensation.
A simple passive heave compensator is a soft spring which effectively strokes in and out in response to string loads as the vessel heaves up and down while effectively holding constant tension on the string. Exemplary types of simple passive heave compensators are crown-mounted compensators or drill string compensators. Passive heave compensators employ hydraulic cylinders and associated gas accumulators to store and dissipate the wave energy.
Active heave compensation differs from passive heave compensation by having an externally powered control system that actively tries to compensate for any movement at a specific point. Exemplary types of active heave compensation include active heave draw works which employ electric or hydraulic winch systems to raise and lower the top drive in response to the vessel motion.
Active and passive heave compensation systems may be combined to provide semi-active heave compensation systems.
The essential nature of the heave compensation function to a floating rig is such that safety standards also demand that they be designed such that no single component failure shall lead to overall failure of the system. They should also be “fail to safety” meaning that in the event of any failure the system defaults to a compensating state, which is the safest state during locked-to-bottom operations. While active heave draw works have numerous benefits, they fail to a “locked condition”. Safe operations and industry standards require additional means of safety to be implemented in the system/equipment configuration. Additional means of safety may include an in-line tensioner, design of a weak link in the riser/landing string, limiting operation parameters to be within the stretch limit of the riser, and so forth.
Generally, these operating parameters place constraints on operators which have direct impact on productivity and efficiency. All these existing options have limitations. In the case of an inline tensioner there are concerns about the how the inline tensioner behaves when run in series with the active heave draw works. In the case of the weak link in the riser, they typically only provide protection in an over-tensioned case and once broken, they provide no support to the landing string thereafter. In the case of limiting operating parameters to within the stretch of the riser, this can impose considerable downtime during offshore operations.
There is therefore a need for an alternative or improved heave compensation apparatus which may operate as a primary heave compensator or as a back-up to the rig's drill string compensator in the event of failure or disablement of the rig's drill string compensator.
There is also a need for an improved heave compensation apparatus which can be used as a lift frame for the installation of pressure control equipment during well testing/well intervention work, as those components are installed in the congested space of the drilling derrick.
The above references to background art do not constitute an admission that the art forms a part of the common general knowledge of a person of ordinary skill in the art. The above references are also not intended to limit the application of the heave compensation and tensioning apparatus as disclosed herein.